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1 | ============== | |
2 | Memory Hotplug | |
3 | ============== | |
4 | ||
5 | :Created: Jul 28 2007 | |
6 | :Updated: Add description of notifier of memory hotplug: Oct 11 2007 | |
7 | ||
8 | This document is about memory hotplug including how-to-use and current status. | |
9 | Because Memory Hotplug is still under development, contents of this text will | |
10 | be changed often. | |
11 | ||
12 | .. CONTENTS | |
13 | ||
14 | 1. Introduction | |
15 | 1.1 purpose of memory hotplug | |
16 | 1.2. Phases of memory hotplug | |
17 | 1.3. Unit of Memory online/offline operation | |
18 | 2. Kernel Configuration | |
19 | 3. sysfs files for memory hotplug | |
20 | 4. Physical memory hot-add phase | |
21 | 4.1 Hardware(Firmware) Support | |
22 | 4.2 Notify memory hot-add event by hand | |
23 | 5. Logical Memory hot-add phase | |
24 | 5.1. State of memory | |
25 | 5.2. How to online memory | |
26 | 6. Logical memory remove | |
27 | 6.1 Memory offline and ZONE_MOVABLE | |
28 | 6.2. How to offline memory | |
29 | 7. Physical memory remove | |
30 | 8. Memory hotplug event notifier | |
31 | 9. Future Work List | |
32 | ||
33 | ||
34 | .. note:: | |
35 | ||
36 | (1) x86_64's has special implementation for memory hotplug. | |
37 | This text does not describe it. | |
38 | (2) This text assumes that sysfs is mounted at /sys. | |
39 | ||
40 | ||
41 | Introduction | |
42 | ============ | |
43 | ||
44 | purpose of memory hotplug | |
45 | ------------------------- | |
46 | ||
47 | Memory Hotplug allows users to increase/decrease the amount of memory. | |
48 | Generally, there are two purposes. | |
49 | ||
50 | (A) For changing the amount of memory. | |
51 | This is to allow a feature like capacity on demand. | |
52 | (B) For installing/removing DIMMs or NUMA-nodes physically. | |
53 | This is to exchange DIMMs/NUMA-nodes, reduce power consumption, etc. | |
54 | ||
55 | (A) is required by highly virtualized environments and (B) is required by | |
56 | hardware which supports memory power management. | |
57 | ||
58 | Linux memory hotplug is designed for both purpose. | |
59 | ||
60 | ||
61 | Phases of memory hotplug | |
62 | ------------------------ | |
63 | ||
64 | There are 2 phases in Memory Hotplug: | |
65 | ||
66 | 1) Physical Memory Hotplug phase | |
67 | 2) Logical Memory Hotplug phase. | |
68 | ||
69 | The First phase is to communicate hardware/firmware and make/erase | |
70 | environment for hotplugged memory. Basically, this phase is necessary | |
71 | for the purpose (B), but this is good phase for communication between | |
72 | highly virtualized environments too. | |
73 | ||
74 | When memory is hotplugged, the kernel recognizes new memory, makes new memory | |
75 | management tables, and makes sysfs files for new memory's operation. | |
76 | ||
77 | If firmware supports notification of connection of new memory to OS, | |
78 | this phase is triggered automatically. ACPI can notify this event. If not, | |
79 | "probe" operation by system administration is used instead. | |
80 | (see :ref:`memory_hotplug_physical_mem`). | |
81 | ||
82 | Logical Memory Hotplug phase is to change memory state into | |
83 | available/unavailable for users. Amount of memory from user's view is | |
84 | changed by this phase. The kernel makes all memory in it as free pages | |
85 | when a memory range is available. | |
86 | ||
87 | In this document, this phase is described as online/offline. | |
88 | ||
89 | Logical Memory Hotplug phase is triggered by write of sysfs file by system | |
90 | administrator. For the hot-add case, it must be executed after Physical Hotplug | |
91 | phase by hand. | |
92 | (However, if you writes udev's hotplug scripts for memory hotplug, these | |
93 | phases can be execute in seamless way.) | |
94 | ||
95 | ||
96 | Unit of Memory online/offline operation | |
97 | --------------------------------------- | |
98 | ||
99 | Memory hotplug uses SPARSEMEM memory model which allows memory to be divided | |
100 | into chunks of the same size. These chunks are called "sections". The size of | |
101 | a memory section is architecture dependent. For example, power uses 16MiB, ia64 | |
102 | uses 1GiB. | |
103 | ||
104 | Memory sections are combined into chunks referred to as "memory blocks". The | |
105 | size of a memory block is architecture dependent and represents the logical | |
106 | unit upon which memory online/offline operations are to be performed. The | |
107 | default size of a memory block is the same as memory section size unless an | |
108 | architecture specifies otherwise. (see :ref:`memory_hotplug_sysfs_files`.) | |
109 | ||
110 | To determine the size (in bytes) of a memory block please read this file: | |
111 | ||
112 | /sys/devices/system/memory/block_size_bytes | |
113 | ||
114 | ||
115 | Kernel Configuration | |
116 | ==================== | |
117 | ||
118 | To use memory hotplug feature, kernel must be compiled with following | |
119 | config options. | |
120 | ||
121 | - For all memory hotplug: | |
122 | - Memory model -> Sparse Memory (CONFIG_SPARSEMEM) | |
123 | - Allow for memory hot-add (CONFIG_MEMORY_HOTPLUG) | |
124 | ||
125 | - To enable memory removal, the following are also necessary: | |
126 | - Allow for memory hot remove (CONFIG_MEMORY_HOTREMOVE) | |
127 | - Page Migration (CONFIG_MIGRATION) | |
128 | ||
129 | - For ACPI memory hotplug, the following are also necessary: | |
130 | - Memory hotplug (under ACPI Support menu) (CONFIG_ACPI_HOTPLUG_MEMORY) | |
131 | - This option can be kernel module. | |
132 | ||
133 | - As a related configuration, if your box has a feature of NUMA-node hotplug | |
134 | via ACPI, then this option is necessary too. | |
135 | ||
136 | - ACPI0004,PNP0A05 and PNP0A06 Container Driver (under ACPI Support menu) | |
137 | (CONFIG_ACPI_CONTAINER). | |
138 | ||
139 | This option can be kernel module too. | |
140 | ||
141 | ||
142 | .. _memory_hotplug_sysfs_files: | |
143 | ||
144 | sysfs files for memory hotplug | |
145 | ============================== | |
146 | ||
147 | All memory blocks have their device information in sysfs. Each memory block | |
148 | is described under /sys/devices/system/memory as: | |
149 | ||
150 | /sys/devices/system/memory/memoryXXX | |
151 | (XXX is the memory block id.) | |
152 | ||
153 | For the memory block covered by the sysfs directory. It is expected that all | |
154 | memory sections in this range are present and no memory holes exist in the | |
155 | range. Currently there is no way to determine if there is a memory hole, but | |
156 | the existence of one should not affect the hotplug capabilities of the memory | |
157 | block. | |
158 | ||
159 | For example, assume 1GiB memory block size. A device for a memory starting at | |
160 | 0x100000000 is /sys/device/system/memory/memory4:: | |
161 | ||
162 | (0x100000000 / 1Gib = 4) | |
163 | ||
164 | This device covers address range [0x100000000 ... 0x140000000) | |
165 | ||
166 | Under each memory block, you can see 5 files: | |
167 | ||
168 | - /sys/devices/system/memory/memoryXXX/phys_index | |
169 | - /sys/devices/system/memory/memoryXXX/phys_device | |
170 | - /sys/devices/system/memory/memoryXXX/state | |
171 | - /sys/devices/system/memory/memoryXXX/removable | |
172 | - /sys/devices/system/memory/memoryXXX/valid_zones | |
173 | ||
174 | =================== ============================================================ | |
175 | ``phys_index`` read-only and contains memory block id, same as XXX. | |
176 | ``state`` read-write | |
177 | ||
178 | - at read: contains online/offline state of memory. | |
179 | - at write: user can specify "online_kernel", | |
180 | ||
181 | "online_movable", "online", "offline" command | |
182 | which will be performed on all sections in the block. | |
183 | ``phys_device`` read-only: designed to show the name of physical memory | |
184 | device. This is not well implemented now. | |
185 | ``removable`` read-only: contains an integer value indicating | |
186 | whether the memory block is removable or not | |
187 | removable. A value of 1 indicates that the memory | |
188 | block is removable and a value of 0 indicates that | |
189 | it is not removable. A memory block is removable only if | |
190 | every section in the block is removable. | |
191 | ``valid_zones`` read-only: designed to show which zones this memory block | |
192 | can be onlined to. | |
193 | ||
194 | The first column shows it`s default zone. | |
195 | ||
196 | "memory6/valid_zones: Normal Movable" shows this memoryblock | |
197 | can be onlined to ZONE_NORMAL by default and to ZONE_MOVABLE | |
198 | by online_movable. | |
199 | ||
200 | "memory7/valid_zones: Movable Normal" shows this memoryblock | |
201 | can be onlined to ZONE_MOVABLE by default and to ZONE_NORMAL | |
202 | by online_kernel. | |
203 | =================== ============================================================ | |
204 | ||
205 | .. note:: | |
206 | ||
207 | These directories/files appear after physical memory hotplug phase. | |
208 | ||
209 | If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed | |
210 | via symbolic links located in the /sys/devices/system/node/node* directories. | |
211 | ||
212 | For example: | |
213 | /sys/devices/system/node/node0/memory9 -> ../../memory/memory9 | |
214 | ||
215 | A backlink will also be created: | |
216 | /sys/devices/system/memory/memory9/node0 -> ../../node/node0 | |
217 | ||
218 | .. _memory_hotplug_physical_mem: | |
219 | ||
220 | Physical memory hot-add phase | |
221 | ============================= | |
222 | ||
223 | Hardware(Firmware) Support | |
224 | -------------------------- | |
225 | ||
226 | On x86_64/ia64 platform, memory hotplug by ACPI is supported. | |
227 | ||
228 | In general, the firmware (ACPI) which supports memory hotplug defines | |
229 | memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80, | |
230 | Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev | |
231 | script. This will be done automatically. | |
232 | ||
233 | But scripts for memory hotplug are not contained in generic udev package(now). | |
234 | You may have to write it by yourself or online/offline memory by hand. | |
235 | Please see :ref:`memory_hotplug_how_to_online_memory` and | |
236 | :ref:`memory_hotplug_how_to_offline_memory`. | |
237 | ||
238 | If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004", | |
239 | "PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler | |
240 | calls hotplug code for all of objects which are defined in it. | |
241 | If memory device is found, memory hotplug code will be called. | |
242 | ||
243 | ||
244 | Notify memory hot-add event by hand | |
245 | ----------------------------------- | |
246 | ||
247 | On some architectures, the firmware may not notify the kernel of a memory | |
248 | hotplug event. Therefore, the memory "probe" interface is supported to | |
249 | explicitly notify the kernel. This interface depends on | |
250 | CONFIG_ARCH_MEMORY_PROBE and can be configured on powerpc, sh, and x86 | |
251 | if hotplug is supported, although for x86 this should be handled by ACPI | |
252 | notification. | |
253 | ||
254 | Probe interface is located at | |
255 | /sys/devices/system/memory/probe | |
256 | ||
257 | You can tell the physical address of new memory to the kernel by:: | |
258 | ||
259 | % echo start_address_of_new_memory > /sys/devices/system/memory/probe | |
260 | ||
261 | Then, [start_address_of_new_memory, start_address_of_new_memory + | |
262 | memory_block_size] memory range is hot-added. In this case, hotplug script is | |
263 | not called (in current implementation). You'll have to online memory by | |
264 | yourself. Please see :ref:`memory_hotplug_how_to_online_memory`. | |
265 | ||
266 | ||
267 | Logical Memory hot-add phase | |
268 | ============================ | |
269 | ||
270 | State of memory | |
271 | --------------- | |
272 | ||
273 | To see (online/offline) state of a memory block, read 'state' file:: | |
274 | ||
275 | % cat /sys/device/system/memory/memoryXXX/state | |
276 | ||
277 | ||
278 | - If the memory block is online, you'll read "online". | |
279 | - If the memory block is offline, you'll read "offline". | |
280 | ||
281 | ||
282 | .. _memory_hotplug_how_to_online_memory: | |
283 | ||
284 | How to online memory | |
285 | -------------------- | |
286 | ||
287 | When the memory is hot-added, the kernel decides whether or not to "online" | |
288 | it according to the policy which can be read from "auto_online_blocks" file:: | |
289 | ||
290 | % cat /sys/devices/system/memory/auto_online_blocks | |
291 | ||
292 | The default depends on the CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE kernel config | |
293 | option. If it is disabled the default is "offline" which means the newly added | |
294 | memory is not in a ready-to-use state and you have to "online" the newly added | |
295 | memory blocks manually. Automatic onlining can be requested by writing "online" | |
296 | to "auto_online_blocks" file:: | |
297 | ||
298 | % echo online > /sys/devices/system/memory/auto_online_blocks | |
299 | ||
300 | This sets a global policy and impacts all memory blocks that will subsequently | |
301 | be hotplugged. Currently offline blocks keep their state. It is possible, under | |
302 | certain circumstances, that some memory blocks will be added but will fail to | |
303 | online. User space tools can check their "state" files | |
304 | (/sys/devices/system/memory/memoryXXX/state) and try to online them manually. | |
305 | ||
306 | If the automatic onlining wasn't requested, failed, or some memory block was | |
307 | offlined it is possible to change the individual block's state by writing to the | |
308 | "state" file:: | |
309 | ||
310 | % echo online > /sys/devices/system/memory/memoryXXX/state | |
311 | ||
312 | This onlining will not change the ZONE type of the target memory block, | |
313 | If the memory block doesn't belong to any zone an appropriate kernel zone | |
314 | (usually ZONE_NORMAL) will be used unless movable_node kernel command line | |
315 | option is specified when ZONE_MOVABLE will be used. | |
316 | ||
317 | You can explicitly request to associate it with ZONE_MOVABLE by:: | |
318 | ||
319 | % echo online_movable > /sys/devices/system/memory/memoryXXX/state | |
320 | ||
321 | .. note:: current limit: this memory block must be adjacent to ZONE_MOVABLE | |
322 | ||
323 | Or you can explicitly request a kernel zone (usually ZONE_NORMAL) by:: | |
324 | ||
325 | % echo online_kernel > /sys/devices/system/memory/memoryXXX/state | |
326 | ||
327 | .. note:: current limit: this memory block must be adjacent to ZONE_NORMAL | |
328 | ||
329 | An explicit zone onlining can fail (e.g. when the range is already within | |
330 | and existing and incompatible zone already). | |
331 | ||
332 | After this, memory block XXX's state will be 'online' and the amount of | |
333 | available memory will be increased. | |
334 | ||
335 | This may be changed in future. | |
336 | ||
337 | ||
338 | ||
339 | Logical memory remove | |
340 | ===================== | |
341 | ||
342 | Memory offline and ZONE_MOVABLE | |
343 | ------------------------------- | |
344 | ||
345 | Memory offlining is more complicated than memory online. Because memory offline | |
346 | has to make the whole memory block be unused, memory offline can fail if | |
347 | the memory block includes memory which cannot be freed. | |
348 | ||
349 | In general, memory offline can use 2 techniques. | |
350 | ||
351 | (1) reclaim and free all memory in the memory block. | |
352 | (2) migrate all pages in the memory block. | |
353 | ||
354 | In the current implementation, Linux's memory offline uses method (2), freeing | |
355 | all pages in the memory block by page migration. But not all pages are | |
356 | migratable. Under current Linux, migratable pages are anonymous pages and | |
357 | page caches. For offlining a memory block by migration, the kernel has to | |
358 | guarantee that the memory block contains only migratable pages. | |
359 | ||
360 | Now, a boot option for making a memory block which consists of migratable pages | |
361 | is supported. By specifying "kernelcore=" or "movablecore=" boot option, you can | |
362 | create ZONE_MOVABLE...a zone which is just used for movable pages. | |
363 | (See also Documentation/admin-guide/kernel-parameters.rst) | |
364 | ||
365 | Assume the system has "TOTAL" amount of memory at boot time, this boot option | |
366 | creates ZONE_MOVABLE as following. | |
367 | ||
368 | 1) When kernelcore=YYYY boot option is used, | |
369 | Size of memory not for movable pages (not for offline) is YYYY. | |
370 | Size of memory for movable pages (for offline) is TOTAL-YYYY. | |
371 | ||
372 | 2) When movablecore=ZZZZ boot option is used, | |
373 | Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ. | |
374 | Size of memory for movable pages (for offline) is ZZZZ. | |
375 | ||
376 | .. note:: | |
377 | ||
378 | Unfortunately, there is no information to show which memory block belongs | |
379 | to ZONE_MOVABLE. This is TBD. | |
380 | ||
381 | .. _memory_hotplug_how_to_offline_memory: | |
382 | ||
383 | How to offline memory | |
384 | --------------------- | |
385 | ||
386 | You can offline a memory block by using the same sysfs interface that was used | |
387 | in memory onlining:: | |
388 | ||
389 | % echo offline > /sys/devices/system/memory/memoryXXX/state | |
390 | ||
391 | If offline succeeds, the state of the memory block is changed to be "offline". | |
392 | If it fails, some error core (like -EBUSY) will be returned by the kernel. | |
393 | Even if a memory block does not belong to ZONE_MOVABLE, you can try to offline | |
394 | it. If it doesn't contain 'unmovable' memory, you'll get success. | |
395 | ||
396 | A memory block under ZONE_MOVABLE is considered to be able to be offlined | |
397 | easily. But under some busy state, it may return -EBUSY. Even if a memory | |
398 | block cannot be offlined due to -EBUSY, you can retry offlining it and may be | |
399 | able to offline it (or not). (For example, a page is referred to by some kernel | |
400 | internal call and released soon.) | |
401 | ||
402 | Consideration: | |
403 | Memory hotplug's design direction is to make the possibility of memory | |
404 | offlining higher and to guarantee unplugging memory under any situation. But | |
405 | it needs more work. Returning -EBUSY under some situation may be good because | |
406 | the user can decide to retry more or not by himself. Currently, memory | |
407 | offlining code does some amount of retry with 120 seconds timeout. | |
408 | ||
409 | Physical memory remove | |
410 | ====================== | |
411 | ||
412 | Need more implementation yet.... | |
413 | - Notification completion of remove works by OS to firmware. | |
414 | - Guard from remove if not yet. | |
415 | ||
416 | Memory hotplug event notifier | |
417 | ============================= | |
418 | ||
419 | Hotplugging events are sent to a notification queue. | |
420 | ||
421 | There are six types of notification defined in include/linux/memory.h: | |
422 | ||
423 | MEM_GOING_ONLINE | |
424 | Generated before new memory becomes available in order to be able to | |
425 | prepare subsystems to handle memory. The page allocator is still unable | |
426 | to allocate from the new memory. | |
427 | ||
428 | MEM_CANCEL_ONLINE | |
429 | Generated if MEMORY_GOING_ONLINE fails. | |
430 | ||
431 | MEM_ONLINE | |
432 | Generated when memory has successfully brought online. The callback may | |
433 | allocate pages from the new memory. | |
434 | ||
435 | MEM_GOING_OFFLINE | |
436 | Generated to begin the process of offlining memory. Allocations are no | |
437 | longer possible from the memory but some of the memory to be offlined | |
438 | is still in use. The callback can be used to free memory known to a | |
439 | subsystem from the indicated memory block. | |
440 | ||
441 | MEM_CANCEL_OFFLINE | |
442 | Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from | |
443 | the memory block that we attempted to offline. | |
444 | ||
445 | MEM_OFFLINE | |
446 | Generated after offlining memory is complete. | |
447 | ||
448 | A callback routine can be registered by calling:: | |
449 | ||
450 | hotplug_memory_notifier(callback_func, priority) | |
451 | ||
452 | Callback functions with higher values of priority are called before callback | |
453 | functions with lower values. | |
454 | ||
455 | A callback function must have the following prototype:: | |
456 | ||
457 | int callback_func( | |
458 | struct notifier_block *self, unsigned long action, void *arg); | |
459 | ||
460 | The first argument of the callback function (self) is a pointer to the block | |
461 | of the notifier chain that points to the callback function itself. | |
462 | The second argument (action) is one of the event types described above. | |
463 | The third argument (arg) passes a pointer of struct memory_notify:: | |
464 | ||
465 | struct memory_notify { | |
466 | unsigned long start_pfn; | |
467 | unsigned long nr_pages; | |
468 | int status_change_nid_normal; | |
469 | int status_change_nid_high; | |
470 | int status_change_nid; | |
471 | } | |
472 | ||
473 | - start_pfn is start_pfn of online/offline memory. | |
474 | - nr_pages is # of pages of online/offline memory. | |
475 | - status_change_nid_normal is set node id when N_NORMAL_MEMORY of nodemask | |
476 | is (will be) set/clear, if this is -1, then nodemask status is not changed. | |
477 | - status_change_nid_high is set node id when N_HIGH_MEMORY of nodemask | |
478 | is (will be) set/clear, if this is -1, then nodemask status is not changed. | |
479 | - status_change_nid is set node id when N_MEMORY of nodemask is (will be) | |
480 | set/clear. It means a new(memoryless) node gets new memory by online and a | |
481 | node loses all memory. If this is -1, then nodemask status is not changed. | |
482 | ||
483 | If status_changed_nid* >= 0, callback should create/discard structures for the | |
484 | node if necessary. | |
485 | ||
486 | The callback routine shall return one of the values | |
487 | NOTIFY_DONE, NOTIFY_OK, NOTIFY_BAD, NOTIFY_STOP | |
488 | defined in include/linux/notifier.h | |
489 | ||
490 | NOTIFY_DONE and NOTIFY_OK have no effect on the further processing. | |
491 | ||
492 | NOTIFY_BAD is used as response to the MEM_GOING_ONLINE, MEM_GOING_OFFLINE, | |
493 | MEM_ONLINE, or MEM_OFFLINE action to cancel hotplugging. It stops | |
494 | further processing of the notification queue. | |
495 | ||
496 | NOTIFY_STOP stops further processing of the notification queue. | |
497 | ||
498 | Future Work | |
499 | =========== | |
500 | ||
501 | - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like | |
502 | sysctl or new control file. | |
503 | - showing memory block and physical device relationship. | |
504 | - test and make it better memory offlining. | |
505 | - support HugeTLB page migration and offlining. | |
506 | - memmap removing at memory offline. | |
507 | - physical remove memory. |